It is known to produce soft solder powder by subjecting a solder melt to flow dispersion in liquids in rapidly rotating agitators.
So for instance D 237 575 A3 describes a method for producing solder paste, whereby a solder carrier consisting of colophonium, and organic solvent, a compound with reducing action and triethanolamine is mixed with solder metal. In a receiver which can be selectively cooled or heated with an agitator the solder carrier is produced at 50.degree. C. by stirring. The solder metal is added in compact form to the receiver, while heating the latter to a temperature which exceeds the melting point of the solder metal by approximately 10.degree. C. and the melted mass is dispersed by being agitated at a high speed of approximately 10000 rpm. After that it is cooled down to approximately 20.degree. C. below the melting point of the solder metal and the agitator is operated at a lower speed until it is cooled to room temperature. This known process has the disadvantage that the obtained particle size of approximately 150 .mu.m is not a fine metal powder. The dispersed solder particles have also different diameters, i.e. they have by far a too broad grain distribution range. Therefore the known method has not proven itself on a large industrial scale, especially because it does not work continuously.
It is also known to use shearing devices working according to the rotor/stator principle for the production of emulsions (liquid/liquid) and suspensions (solid/liquid) (see IKA Maschinenbau- Prospekt "Dispergieren", pages 22-24, 1997). These devices are used for lacquers, dyes, pharmaceutical products, metal oxide suspensions and coatings. According to this known principle, as a rule, it has to be insured that in the case of highly viscous media the media flow has to be sustained by conveyor units.
Furthermore from DE 44 02 042 A1 a process is known for producing microparticulate reflow-solder agents, whose solder metal content is present in a small grain size range. The compact solder metal is melted into an organic liquid which can be heated to high temperature, such as castor oil, and by means of a flow dispersion process, brought to a spherical symmetrical grain size range of preferably 3 to 10 .mu.m in diameter. The organic liquid is then removed to the extent that the metal particulate remains covered, so that it can be introduced in an emulsion and the individual particles of the suspension and emulsion are covered according to the method of complex coacervation with a melamine polymerisate within the layer thickness range of 50 to 250 nm. The microparticulate organic phase is then quantitatively separated from the microparticulated metal phase. This microparticulate metal powders are protected by a duroplastic polymer system, however they can be released again only through the action of a highly activated fluxing agent. These fluxing agents lead to the destruction of the microelectronic switching circuits and are therefore unsuitable. Besides this method has been used only in laboratories and is not capable of insuring a uniform sphere diameter from charge to charge.
Another known solution (U.S. Pat. No. 4,648,820) melts metal such as aluminum in a crucible, feeds the molten metal to a cooling chamber filled with cooling fluid, and disperses the liquid metal by means of spinning disks in drops, which again are drawn together with the cooling fluid into a recirculation cycle and in a separator are separated from the cooling fluid, whereby the latter is returned to the cooling chamber.
According to U.S. Pat. No. 5,411,602 the solder is melted and the molten solder is divided into drops by means of inert gas. This state of the art is also plagued by the drawback that the produced metal particles do not have uniform sphere diameters, so that in any case sorting processes are necessary in order to select metal particles of an approximately equal size having the same sphere diameter. That renders this known solution inefficient.